Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
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FXUS61 KRNK 222349
Area Forecast Discussion
National Weather Service Blacksburg VA
649 PM EST Sun Jan 22 2017
A strong area of low pressure over the Mid Mississippi valley
will approach from the west tonight bringing a period of
moderate to heavy rain. This low pressure system will slide
slowly east across across the area Monday before exiting Monday
night. In the wake of the low pressure system, a period of gusty
northwest winds are possible for the North Carolina high country
and the Blue Ridge. Temperatures trend closer to seasonal
levels by later in the week.
.NEAR TERM /THROUGH MONDAY/...
As of 337 PM EST Sunday...
Conditions are relatively quiet as of this writing as a
persistent surface wedge/CAD has kept us overcast and cool
(though still above normal); however, that stands to change
abruptly tonight and extending into much of the short-term
Regional surface analysis depicts a developing low (now at 992
mb in central Alabama); its surface warm front extending
eastward into coastal Georgia/SC with a frontal occlusion south
of the surface low just west of Talahassee FL. Surface cyclone
is projected to deepen further and advance northeast tonight
into the southern Appalachians, and move more slowly east into
the eastern shore of Virginia/North Carolina Monday. Aloft,
pronounced closed upper low over the lower Mississippi Valley
will shift northeast into western North Carolina by morning,
eventually becoming nearly vertically stacked with the surface
cyclone. Slow northeast progression is anticipated into eastern
Virginia from 12z Monday thru 00z Tuesday.
For Tonight: Active period with several aspects to consider,
including rainfall/hydro aspects, breezy east to northeast winds
at elevation and potential for elevated thunder tonight.
Regarding the rainfall aspect, warm front/occlusion pivots
northeast over top of lingering wedge late tonight and into
early overnight. Most of the higher resolution CAM models and
coarser-res global models indicate that the eastern two-thirds
of the forecast area stands to get brushed by at least steady
rain on the northern "comma-head". This rainfall will probably
sharpen owing to strong 850-700 mb frontogenetic forcing
apparent in the 12Z GFS. Given 40 kts of easterly flow tapping
into higher PWATs over the western Atlantic and the GFS
indicating marginal but still noticeable instability (elevated
instability with MUCAPEs up to 150 J/kg), I`ve added heavy
rainfall wording into the forecast for an area from the Virginia
and North Carolina piedmont into the southside of Virginia area
for a part of the overnight. It`s such a narrow corridor of
higher moisture content, though, that I didn`t extend it further
west than those areas, but even areas to the Blue Ridge stand
to see steady light to moderate rain and slight chance for
Thunder. But the risk is slight in every sense of the word and
we are not expecting even stronger thunderstorms at this point
given the meager degree of instability. In fact, SPC has trimmed
back its earlier areas of Marginal/Slight Risk across parts of
our forecast area to indicate more general thunder, which seems
reasonable. Steadiest rain doesn`t get into the Mountain Empire
northeast into southeast West Virginia and the Alleghany
Highlands until well into the overnight as the precipitation
shield pivots back northwestward along an inverted trough
extending north from the upper low. Precipitation gradually
becomes more showery east of the Blue Ridge as that process
proceeds. So by the morning commute, the steadiest rain would be
confined to our western zones. Wide range in forecast QPF
amounts ranging from up to a tenth in western Greenbrier county
to near 1.5 inches along the North Carolina high country up into
the Virginia Piedmont. Rainfall rates should generally be
steady/gradual, but potential for slightly higher rates tonight
in the southside of Virginia and Virginia/North Carolina
Specific to winds - easterly winds stand to increase both at
the surface but particularly at elevation. East-northeast low
level jet of up to 50 kts progresses up the spine of the Blue
Ridge, with gusts up to 35 mph above 3000`. At this point, given
the potential for moderate rain and that it is not a common
trajectory for strong winds, opted to hold on any wind headlines
due to these easterly winds.
Under overcast, lows will generally not fall all that much and
I`ve generally warmed temps from MOS by several degrees, into
the 40s to low 50s.
For Monday: Steady light rain will be ongoing across the western
portion of the forecast area (Blue Ridge westward) with high-
Likely to Categorical PoPs through much of Monday, focused by an
axis of mid-level deformation on the northern side of the upper
low. Precipitation character east of the Blue Ridge should stay
more showery however as a dry slot on the eastern side of the
upper low pushes northward across that portion of the forecast
area. Though the NAM offers some leftover instability east of
the Blue Ridge, I`m not as convinced such will be realized as it
will take a break in the clouds to really tap into that
instability. Rainfall amounts Monday are greatest west of the
Blue Ridge (areal amounts of a quarter to half inch) with up to
a tenth of an inch east. By late in the day as surface low exits
into the Piedmont, winds begin to shift from light and variable
to northwest with an increase in speed, and may turn gusty at
elevation late in the period. Temps range from the upper 40s to
the middle 50s across the forecast area.
.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 337 PM EST Sunday...
Stacked low pressure system will be exiting east and offshore Monday
night with lingering precip mainly of the wrap around/upslope variety
espcly western mountains with decreasing coverage east overnight as
subsidence/downslope develops. Will again have the higher likely pops
west of the Blue Ridge along whats left of the deformation zone early
on and strong northwest upslope through early Tuesday. Appears may
finally get cold enough aloft under the passing upper cold pool and
cooling boundary layer to support at least some elevation snow showers
espcly BLF-LWB corridor and points west. However wont go as cold as the
latest Nam as it has a much colder conveyor belt scenario per the low
tracking farther south to start. This still supports an inch or so of
snow northwest ridges before moisture shallows out from above by
Tuesday afternoon when will cut pops to just slight pops.
Other lingering concern again with the potential for very strong
northwest winds in the wake of the system Monday night into Tuesday
afternoon as the pressure gradient ramps up on the back of exiting
upper low. Latest GFS still showing 45-50 kts at 850 mb with a period
of strong subsidence and lowering inversion levels overnight. This in
conjunction with local scheme program values supports advisory criteria
along the Blue Ridge from TNB to ROA so may expand mention in the HWO.
Quiet weather returns Tuesday night into Wednesday as strong shortwave
ridging shifts east ahead of the next upstream cold front. Very warm
temps aloft under this feature likely to bring the last of the mild
days for a while Wednesday with potential for all spots to break 60,
and perhaps near 70 Piedmont given westerly flow, dry air and sunshine.
Next cold front enters from the west Wednesday night signaling the end
of the long duration thaw as cooling 850 mb temps chase the boundary
east overnight. Some solutions attempting to link some southern stream
moisture up along the boundary overnight similar to the GFS, while
others tending to keep jet moisture sheared and separate. Again think
the GFS is overdone but for consistency sake will leave in some low
pops overnight. This mostly rain showers until perhaps very late far
west where colder air could catch departing moisture with a brief
period of snow showers before upslope kicks in. Lows mostly 30s west to
low/mid 40s east.
.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 337 PM EST Sunday...
Pattern change to overall broad longwave troughing over the region will
begin to take shape Thursday in the wake of a passing surface cold
front by early in the day. This will lead to a prolonged period of cold
advection under west/northwest flow to the east of the main surface
high situated over the Rockies through next weekend. However appears
the overall scenario to remain on the quiet side with mainly shallow
moisture resulting in upslope driven periodic snow showers espcly
northwest sections into Sunday. Appears amounts to be limited though
given more westerly trajectory and only shallow moisture beneath the
inversion, so overall only chance pops this far out along the western
slopes. Otherwise should see the typical split from mainly sunny/clear
skies east of the ridges per downslope, to clouds far west, and at
times out to the Blue Ridge pending the passage of weak impulses.
With the broad trough becoming rather flat and 850 mb temps roughly
above minus 10C, only expecting a return to normal or slightly below
normal temps with highs 30s/low 40s mountains, to low/mid 40s east, and
lows in the 20s. However will feel quite a bit colder after all the
recent warmth, given a persistent blustery northwest wind under the
lingering gradient between low pressure well north and the high well to
the west. Appears coldest could occur during the weekend when thickness
drop a bit so trended toward the colder earlier EC mos as the GFS MEX
values remain warmer.
.AVIATION /00Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --As of 649 PM EST Sunday...
Poor flying conditions will continue across the terminals
through much of the TAF period. Only very slight improvement is
expected by later Monday.
Wedge/CAD conditions will continue generally IFR to LIFR
categories mainly due to low ceilings with intermittent periods
of MVFR/IFR rain.
A strong warm front advances northward across the region
tonight into Monday. Low pressure will consolidate off the
Virginia coast by Monday evening.
Expect a period of at least light to moderate rain (visbys 4-6
SM) across the TAFs, although rain may be heavy at times with
IFR VSBY at DAN overnight. Because any thunderstorms will be
hindered by the stability of the Wedge. Decided to limited
potential for thunder, and have left out of the TAFs.
Nevertheless, IFR to LIFR conditions should continue as the rain
Along/ahead of the warm front, easterly winds increase to near
40 kts above 3000ft. Light easterly winds will increase to near
10-12 kts at most terminals with gusts to 20-25 kts at KBLF,
KLWB and KBCB through early morning. On Monday morning, Warm
frontal rainband pivots north-northwest. Steadiest rain, light
to moderate, to then focus and west of the Blue Ridge, with
intermittent/showery rains at KLYH and KDAN.
Slow improvement anticipated Monday with conditions primarily
IFR to MVFR near the end of the TAF period. Easterly winds veer
around to southeast/south 4-6 kts most terminals, but become
west to northwest at KBCB/KBLF/KLWB late in the TAF period.
The timing of the improvement of ceilings appears a little
slower on this evenings model run.
Medium confidence in ceilings and visibilities throughout the
TAF valid period. Medium confidence in wind speed and direction
throughout the TAF valid period.
Extended Aviation Discussion...
The slow moving and deep upper low will continue to affect the
area Monday with waves of rain, but the thunder threat should
have moved well east of the area by Monday. Expect sub-VFR
conditions to continue into Monday, but the activity will likely
become more showery in nature as the center of the upper low
drifts overhead and to the north of the region. The upper low
will finally move northeast of the region Tuesday resulting
in improving conditions, especially east of the Blue Ridge.
Gusty northwest winds are expected Tuesday as surface low
pressure deepens across the northeast U.S. High pressure will
finally spread into the region by Wednesday. A band of MVFR
showers possible along/behind the front Thursday into Thursday
night. West to northwest flow will continue into the Weekend
with upslope driven periodic snow showers in the western
-- End Changed Discussion --
As of 326 PM EST Sunday...
Periods of rain will impact the Blacksburg hydrologic service
tonight into Monday from a complex frontal system taking shape
over the southern United States.
A warm front moving in tonight from the south will allow for a
widespread period of rain. Areas south and east of the Blue
Ridge stand to see the greatest potential for heavier rainfall
rates overnight tonight. The rain band then pivots northward
where rainfall intensity is expected to be more steady and
lighter. Rainfall should be primarily focused from the Blue
Ridge west during the day on Monday, with more
intermittent/showers along and east of the Blue Ridge.
Forecast rainfall amounts tonight through Monday night range
from 1 to 2 inches.
There have been modest river rises occurring today, and the
expectation continues to be a gradual rise on main-stem river
rises with faster/greater response on smaller rivers/creeks,
along with some potential for ponding on roads and in areas of
poor drainage. The only river forecast point projected to reach
or go just above flood stage (Minor Flooding) is the Dan River
at South Boston. Since this is the only river forecast point and
there is still considerable uncertainty as to how much, if any,
we will exceed 2 inches of rainfall (which is what it would
take to cause flooding), will not issue a Flood Watch at the
present time. Worst case would be to have this 2 inches fall in
a shorter period of time or the upper low slow its forward
progress resulting in a longer duration of upslope flow with
rain amounts exceeding the 2 inches.